1. Field of the Invention
The field of art to which this invention pertains is aromatic polyamide fibers and, more particularly, it is directed to a method of dyeing these fibers.
Specifically, the instant invention is a method of dyeing a fiber structure of poly(p-phenylene terephthalamide (PPD-T) fibers or filaments with a water-soluble cationic dye. The fibers are dyed in a dye bath at a high pressure from about 29 to 108 psi at a temperature of from 130.degree. to 180.degree. C. for a time sufficient to dye the fibers. Preferably such fibers are heated to a temperature in excess of 150.degree. C., during the dyeing operation. These fibers, which are preferably dried prior to being placed in the dye bath, can be dyed as tow or in the form of staple fibers or yarns or fabrics.
By following the method of this invention, dried PPD-T fibers may be efficiently dyed a bright deep color with highly compatible water-soluble dyes without loss of dye effectiveness.
2. Description of the Related Art
Aromatic polyamide fibers are known to the art. They have outstanding properties such as high tensile strength, high modulus, flame and heat resistance, and good flex life which make them suited to be formed into fabrics usable as protective clothing, and for many other uses.
More specifically, this invention is directed to a method of dyeing aromatic polyamide fibers of a poly(p-phenylene terephthalamide) polymer, hereinafter referred to as PPD-T fibers. Such fibers which are described in greater detail in U.S. Pat. Nos. 3,767,756 to Blades; 3,869,429, also to Blades and 4,144,023 to Provost, for example, possess many useful properties, as indicated above. The teachings of these patents are incorporated herein by reference.
It is well-known that aromatic polyamide fibers and particularly PPD-T fibers are difficult to dye. As a result, various methods have evolved, over the years, for dyeing poly(p-phenylene terephthalamide, (PPD-T) fibers in various forms.
The most generally accepted method of dyeing PPD-T fibers has been to dye the fibers, after they have been crimped, in a pressure vessel at 121.degree. to 132.degree. C. temperatures, using an aqueous dyebath containing a large percentage of a carrier, such as acetophenone. This method has proven to be effective for dyeing dried PPD-T fibers with water-soluble dyes. However, the large amount of carrier involved can present certain cost and dyebath disposal problems to the user. Further, several hours are generally required to achieve the depth of color desired in the finished product. Also, this method is more suitable for dyeing fabric or staple fibers.
Another method suitable for dyeing crimped PPD-T fibers is shown in U.S. Pat. No. 4,144,023 to Provost. In this method high strength, high modulus aromatic polyamide PPD-T fibers are dyed by crimping the fibers while wet to at least 10 crimps per inch and maintaining at least 15% by weight moisture based on the dry fibers in the fibers at all times before dyeing. These fibers are preferably dyed at 121.degree. C. under pressure for a number of hours. Effective K/S values (apparent dye depth levels) are obtained using this method. If the fiber is dried before dyeing, a much lower K/S value is obtained. Only a limited number of cationic dyes are suitable for use in this dyeing method.
Accordingly, a method has long been sought for dyeing PPD-T fibers, which have been dried prior to dyeing, using commercially available water-soluble dyes, without the need of a carrier, to obtain a wide range of colors while retaining good fibers properties. It has been especially desired to achieve a method for applying such dyes at relatively low temperatures, e.g., around 165.degree. C. (and under 200.degree. C.), since many otherwise desirable dyes are unstable at higher temperatures. And, it further has been desired to be able to dye PPD-T fibers in staple form or PPD-T fibers in the form of yarns or fabric effectively with cationic dyes using commercially available dyeing equipment.
This invention solves these and other problems found in the prior art by surprisingly finding that dried PPD-T fibers in the form of staple or yarn or fabric may be effectively dyed under high pressure conditions using a wide range of cationic dyes without need for a carrier. Specifically, it has been found that such fibers may be dyed with water-soluble dyes at high pressures of 29 to 108 psi and temperatures from 130.degree. C. to 180.degree. C. These fibers may be dyed to acceptable levels using dyeing equipment and techniques known to the art. In so doing, the method of this invention provides the art with an effective, improved means of dyeing PPD-T fibers with a large variety of water-soluble dyestuff.